Method for etching aluminum and aluminum alloy bodies



March 14, 1961 E. R. HOLMAN 2,975,041

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United States atenr :9

' 2,975,041 Patented Mar. 14, 1961 ice METHOD FOR ETCHING ALUMINUM AND ALUMINUM ALLOY BODIES Emmette R. Holman, Pasadena, Calif., assignor to Purex Corporation, Ltd., a corporation of California Filed Aug. 6, 1956, Ser. No. 602,249

6 Claims. (Cl. 41-42) This invention relates to methods for etching aluminum and aluminum alloy bodies, hereinafter collectively referred to as aluminum.

in the alkali combined as aluminate, and that this procedure causes a precipitation of hydrated alumina" from solution. 1

I have now discovered thatthe presence of excess of alumina as a solid phase in contact with'the cooled soluincorporation ofanygsubstantial.quantities of the alumie nate and thus causing a regeneration of the alkali metal hydroxide used to :.disso.lve:the aluminum; The process is thusjse'lf sustainingas to the employment of sodium hydroxide; and make .uprasfre'quired is in substantial This application ,is a continuation-impart of my ap- V plication Serial No. 449,994, filed August 16, 1954, now U.S. Patent No. 2,884,728.

Commercially aluminum is etched by alkaline solutions for purposes of modifying the surface characteristics. It is also employed to alter the shape or dimensions of aluminum objects, and in such proceduresthe process is a substitute for die forming, forging, machining or other metal cutting and forming process having elements of marked superiority over such procedures.

In the course of such etching processes, the aluminum The actual chemical state of the aluminum is not completely understood but it is believed that it forms a salt which is an aluminate having the constitutional formula (R O).(Al O where R is an alkali metal preferably Na or K. Thus, where sodium hydroxide is used as the etchant, the salt, it is believed, is in the form of the Na O.Al O although whether as the monomer NaAlO or the dimer Na Al O or mixtures thereof is not certain.

During the course of the etching reaction, the free alkali, i.e., the hydroxide not combined with the aluminum in salt formation decreases while the concentration of the aluminum salt in solution increases.

This process results in two disadvantages. First, the rate of attack, i.e., the rate of solution of the metal in the alkaline etching bath falls and, second, a concentration of aluminate is reached in which precipitation of aluminum containing sludge may occur to an objectionable extent.

The precipitation of sludge in the etch tank is highly objectionable and methods have been suggested in the prior art to alleviate this difficulty. Thus, agents have been added to retard the formation of this sludge, and agents have been added to modify the sludge when formed.

In my co-pending application Serial No. 449,994, filed August 16, 1954, sludge formation was minimized in the etch tank, by withdrawing a portion of the solution during etching and settling in a lower temperature zone, the precipitated sludge removed and the clarified solution reheated and recirculated to the etch bath.

While it is known that if the alkalinity of the etch tank falls too low and sludge precipitates that some of the sludge contains hydrated alumina and it has been suggested that this is formed by hydrolysis of the sodium aluminate, the indications are that a portion of the sludge formed contains precipitatedsodium aluminate. Precipitation of aluminate may also occur in alkali having such high concentrations of aluminate in excess of the solubility of the aluminate in such alkaline solutions at the temperature employed.

It has been found that cooling the relatively hot alkaline etching solutions which contain dissolved aluminate, re sults in an increase in alkaline hydroxide and a decrease measure-due to mechanical losses and entrainment.

I may operate thexetching'pr'ocess so that little or no sludge is formed; the level of concentration of the aluminate in solution and the'concentration of the alkali as free alkali being such that no hydrated alumina 'precipitates. 7

Where I operate under: conditions during etching or in the regeneration procedure such that aluminate is precipitated, I provide means for the recovery, and hydrolytic treatment of the aluminate so that it may be converted into free alkali and in part returned, if desired together with the regenerated alkali for further use in an etch tank. I withdraw the etchant and cool the same with or without added water and recirculate to the cooled solution some solid hydrated alumina. By so doing I precipitate along with the added hydrated alumina a material proportion of the aluminum in the solution. a 1

The result of these procedures is an increase'in the free alkali content of-the solutionand a reduction in the concentration of the aluminate in solution. The precipitate thus formed is believed to be alumina trihydrate Al O .3I-I O and will be so referred to throughout this specification and claims. However it will understood that this does not exclude the possibility that some alphamonohydrate may be present. I

My process'comprises contacting an alkaline solution with aluminum to cause-a reduction in the free alkalinity of the solution andyan-increase in the aluminum in solution, it is believed' as aluminate, and then reducing the V temperature of thesolution;

If at this point there is no precipitation or an insubstantial amount of solid phase hydrated alumina is formed; I add suflicient amount of such solid phase to cause a thorough contact between the body of the solution and thedispersed hydrate. Onstanding, the hydrated alumina is precipitated from solution and the clear solution may be separated by settling and decanting. The vsolution. shows an. increase in the free alkali and a decrease in the amountof the aluminate in solution.

In my process the percentage of the aluminum present in solution as aluminate in the feed to the regenerator, and which is regenerated in my process, depends on the concentration-of the free alkali,'expressed as Na o of the feed solution and upon the concentration of the aluminum in solution. expressed as the aluminate Na O.Al O The percentage of the aluminum dissolved in the solution and then precipitated, for any given terminal molar ratio of the total alkali, free and combined, to the aluminum in solution, depends on the magnitude of this ratio in the solution prior to'cooling. The per-' centage increases as the said ratio in the original solution decreases.

.Thus, for any. given final temperature employed in the process of this invention, the further the etching reac-;

tion has progressed and the lower the concentration of free alkali and the greater the concentration of the aluminum in solution, the greater will be the percentage precipitation of the aluminum from solution in the form of hydrated alumina and the greater the ratio of the concentration of free alkali in the regenerated solution to the concentration of the total alkali in the solution, free and combined.

Where the regenerated solution is to be used in forming a new etching bath, I find it convenient not to carry the regeneration to completion but instead I leave in solution a portion of the aluminum. It has been found advantageous for purposes of control of the etching reaction to start the etching bath with a portion of aluminum compound in the solution. To obtain this I limit the degree of regeneration to leave a portion of the aluminum in solution as aluminate.

In operating my process as a continuous process, I may also control the regeneration of the etch bath so that the recirculation of the regenerated solution maintains a substantially constant ratio of free alkali to aluminate in the etch tank during the etch cycle.

Example I 4022 grams of a solution was made by dissolving 422 grams of commercial caustic soda (about 96%, or 405 grams of NaOH) in 3600 grams of distilled water in a tared 8 liter stainless steel beaker. This solution analyzed 10.2% NaOH by the method of Watts and Utley, Analytical Chemistry 25, pp. 864-867 (1953). The solution was heated and maintained at 190-200 F. while weighed pieces of 7075 aluminum alloy were etched by it. The beaker was covered to reduce losses by evaporation and by carry-over of entrained mist produced by the vigorous evolution of hydrogen. When the solution reached the point of exhaustion, as represented by the practical cessation of gassing, the system was restored to its original volume by the addition of distilled water as required.

At this point the solution analyzed 1.4% of free NaOH representing 60 grams of NaOH. The batch was cooled to room temperature and seeded by stirring in 100 grams of AI O .3H O, alumina trihydrate (Alcoa C-730, a product of Aluminum Company of America), having a particle size of less than 0.6 micron. The covered batch was then allowed to cool to ambient room temperature and stand undisturbed for 16 hours (overnight). Next day the clear supernatant liquid was poured off through a large Buchner funnel. The sludge was then transferred to the filter and sucked as dry as practicable. 3137 grams of filtrate was thus recovered. The filtrate analyzed 5.8% free NaOH representing 182 grams of NaOH and showing a regeneration of alkali metal hydroxide of from 1.4% of the solution to 5.8% by weight of the solution.

Example 2 170 grams of commercial caustic soda (about 96%, or 163 grams of NaOH) was dissolved in the same tared beaker containing 3125 grams of the filtrate from Example 1. The solution analyzed 10.7% free NaOH at this point, corresponding to 338 grams of NaOH. The beaker and contents were heated and maintained at 190- 200 F. and saturated with respect to dissolved 7075 aluminum alloy as in Example 1. When the evolution of hydrogen gas virtually ceased, the batch was brought up to volume with distilled water and cooled to room temperature. It showed upon analysis 3.3% of free NaOH, corresponding to 116 grams of NaOH at this point. The batchwas seeded with 100 grams of Al O .3H O, allowed to stand overnight, and filtered as in Example 1. The recovered filtrate, weighing 2143 grams analyzed 7.6% free NaOH.

The above data shows an increase of free hydroxide from 3.3% of the solution to 7.6% of the solution.

Example 3 250 grams of commercial caustic soda (about 96%, or 240 grams of NaOH) was dissolved in 1631 grams of distilled water and added to the tared beaker containing 2130 grams of the filtrate from Example 2. The 4011 grams of new mixture analyzed 10.2% free NaOH, corresponding to 409 grams of NaOH. The beaker and contents were heated to 190-200 F. and saturated with respect to 7075 aluminum alloy, brought up to weight with distilled water and cooled to room temperature as in Example 1. At this point the liquor analyzed 0.3% free NaOH, corresponding to 13 grams of NaOH. The batch was then seeded with 100 grams of Al O .3H O, allowed to stand overnight and filtered as in Example 1. The recovered filtrate, weighing 2593 grams, analyzed 7.4% by weight of free NaOH in the solution. This is an increase from .3% by weight of NaOH in the solution to the regenerator to 7.4% from the generator.

In the case of each of the examples above, the sodium hydroxide, which in the original etching reaction was converted from free sodium hydroxide to sodium aluminate was increased in value by the regeneration and resulted in the formation of sludge formed from the aluminum dissolved in the etching tank. This sludge it is believed is in the form of hydrated aluminum oxide.

By controlling the alkalinity of the solution, for example, so that the ratio of total alkali, free and com bined as aluminate, expressed as mols of Na O in the bath, to the aluminum in solution, expressed as mols of A1 0 is in excess of about 1.1 and preferably 1.5 or

higher, the precipitation of hydrated sludge is inhibited in the etch tank and the aluminum removed by the alkali in the etch process remains in solution as the aluminate. I prefer to operate the etch process under such controls.

In carrying on the process of my invention I may operate in a batch or on a continuous basis. The process may be applied either to etching baths which are operated to precipitate aluminiferous sludge spontaneously in the etch bath or those in which no such sludge precipitates.

In carrying out the process in connection with etching operations in which sludge is formed spontaneously, the sludge containing solution is withdrawn to a separate tank and reduced in temperature and diluted if this is desired. A regeneration of the solution by precipitation of the hydrated alumina occurs. Where the aluminum which is etched contains alloying metals which are soluble in alkaline solutions, for example such as zinc, these metals will also be precipitated, as I believe, in oxide form, along with the aluminum. Usually, at the concentrations employed in the etching reaction insoluble zinc oxide rather than soluble zincate forms in the etching bath. The result will be a substantial increase in the free alkali content of the solution, and such solution can then be recirculated to the etching bath or used as make up in another etch bath.

The precipitated hydrated alumina may be contaminated with precipitated aluminate and this aluminate may be separated from the hydrated alumina and added to the material entering the regeneration stage or separate ly regenerated.

Where the procedure is employed with etching solutions which operate without spontaneous precipitation of alumina hydrate sludge in the etching chamber as is preferred, I prefer to add to the cooled solution prior to, or during, or at the end of the cooling step, in such cases as do not then result in sludge precipitation, an excess of solid hydrated alumina and preferably in finely divided state to cause an intimate contact between the solution and the solid hydrate. By this procedure precipitation of hydrated alumina and regeneration of the solution results, to cause an increase in the concentration of the free alkali in solution.

The precipitated hydrated alumina may then be removed by any conventional clarifyingprocedure and the clear liquor recirculated or employed as described above. Where the procedure is employed in connection with a continuous process, I may withdraw a'portion of the etchant from the etch bath and treat as described above and return the clarified and regenerated solution, adding water and extra alkali and adjusting the temperature to re-establish the concentrations needed for maintenance of substantially constant conditions in'the etch bath.

My invention will be further described by reference to the drawing, which is illustrative only. The drawing is a schematic flow sheet of my process.

The etching reaction is conducted in zone 110, usually in a tank in which the aluminum is suspended. The etch process is conducted with alkali concentrations usually in the neighborhood of -20% at the start ofthe operation, a convenient percentage is about by weight of free sodium hydroxide or equivalent free a1- kali. Temperatures used are from about 150 to 200 F., a convenient temperature being 190 F. In this process as indicated above, the aluminum goes into solution and the free alkali content falls. This procedure is continued, the etched metal being withdrawn, when its etching process is concluded, and fresh metal added. During the operation as the process continues, there is a loss of liquid due to water loss by decomposition, evaporation, as mist due to evolution of hydrogen gas, and as water of crystallization of Al O .3H O during the regeneration. Water may be added to maintain levels during the process.

During the course of this reaction, two forms of sludge may precipitate. One composed, of hydrated alumina or aluminate or mixture thereof as described above and the other containing a substantial amount of compounds of other metals, referred to as non-aluminum sludge. The non-aluminum sludge sometimes referred to as smut, contains some free metal and compounds of the metals alloyed with aluminum. The role of zinc present in some aluminum alloys has been described above.

Where the above procedure is operated so as to cause the spontaneous precipitation of sludge containing the hydrated alumina with or without precipitated aluminate, I need not add additional caustic during etching, but may add water to make up for water loss. Under these conditions, the etching reaction slows down, the caustic is depleted and soon reaches a lower limit, when further use of the solution is uneconomical. .This may occur at a concentration of free caustic of about 2 to 5% by weight extpressed as NaOH, depending on the alloy, the temperature and the etching rate which is thought to be economical.

Where I operate under conditions wherein no spontaneous precipitation of a sludge containing aluminum compounds occurs in the etching tank, I may, and preferably do, first remove any smut which is precipitated in the etch tank prior to regeneration of the solution, as for example, in centrifuges 114 and 115, and I recirculate to the clarified and preferably cool solution some hydrated alumina, for example, such as is precipitated in the process, and pass the mixture to the settling zone. The non-aluminum metal-containing smut or sludge, ineluding non-aluminum metal compounds, so separated, can then be worked for recovery of the non-aluminum values therein.

I may operate the cycle as in the flow sheet illustrated in the drawing in the following manner:

The etch tank is operated at a concentration of free alkali and dissolved aluminum such that substantially no precipitation of aluminum hydrate sludge or sodium aluminate occurs in the tank 110. For treatment of aluminum using sodium hydroxide as the etchant at a temperature of 170190 F. this occurs when the ratio of the total alkali in the etching solution to the aluminum in solution exceeds about 1.2 to about 1.5 gram mols of total alkali hydroxide in the etchant solution to 1 gram 6 atom of aluminum dissolved in the etch solution at the temperatures of the etch process. I

However, as I have found, if such a solution having a sufiiciently high concentration of dissolved aluminum is withdrawn and cooled to a lower temperature, a portion of the aluminum in solution may be precipitated. This precipitation is believed to be in the form. of hydrated alumina and under suitable conditions may be substantially free of aluminate and is facilitated by adding a portion of such hydrated alumina to establish a solid phase in contact with the solution.

The alkaline etching solution at the operating temperature in tank is continuously withdrawn via pump 111 and passed by proper manipulation of valves 112 and 113, 116 and 117 into centrifuges 114 and 115, one centrifuging while the other is discharging. Instead of centrifuges filters may be employed. The clarified hot solution is passed by pump 118 via line 119, valves 120 and 121 being properly adjusted, into continuous contactor 122. If the separation of the smut is not important, the centrifuges may be by-passed via line 113'. The solution is cooled and contacted in 122 with recirculated sludge passed from the settler via line 134 into line 119. To assist in heat recovery, all or a portion of the hot solution passing via line 119 may be passed via line 120' in heat exchange relation with eifluent from the settler 132 in heat exchanger 122 and further cooled in cooler 123 and reintroduced via 131 into line 119. Temperature can be further controlled by means of cooling coils 125 in continuous contactor 122. Additional water via. 152, 157 and 131 or effluent via line 127, 143, 157 and 131 may be introduced via line 119. A portion of the liquor containing precipitated hydrated alumina is re moved from the contactor via line 124 and passed into the continuous settler and thickener 132. The clarified overflow is discharged from the launder 126 via line 127 and pump 128.

The dwell times in the contactor 122, settler 132 and the temperatures employed as well as the further treatment of the precipitates depends on the controls to be maintained in the etch tank 110.

Where it is desired to maintain a relatively low aluminate content in the etch tank and a high degree of conversion of the aluminate formed into alkali metal hydroxide and removal of the alumina from the solution as hydrated alumina I may follow the following cycle illustrated.

The etch solution in tank 110 is circulated at a sufliciently high rate to maintain a relatively low concentration of aluminate in the solution exiting via 113 preferably at concentrations at which, at the temperatures maintained in etch tank 110, in 122, and 132, and at the alkali concentrations maintained in the system, the aluminate is soluble, i.e., does not precipitate as a sludge at the temperatures maintained in the etch tank 110 or in the contactor or thickener. The dwell time in the com tactor 122 and the thickener 132 is sufficient at the temperatures there maintained to cause the desired degree of regeneraion of the alkali metal hydroxide from the aluminate but the temperature is maintained sufliciently. high so that at the concentrations of free hydroxide substantially no precipitation of the aluminates occurs.

To assist in the precipitation of the hydrated alumina and in the regeneration of the alkali, excess of solid phase of hydrated alumina in the contactor 122 is established by introduction of a slurry of the hydrated alumina into line 119 via line 134 as will be more fully described below.

The eflluent via 127 has a higher concentration of free alkali and a lower concentration of aluminate than does the feed via 113. The thickened slurry of hydrated alumina is withdrawn via line 135 and a portion of the slurry may be recirculated via line 134 to line 119 for introduction into the contactor to maintain an excess of solid phase in contact with the solution in contactor 122.

A portion of the thickened slurry of hydrated alumina is passed directly to the filter 136, the filtrate Withdrawn via line 141, and introduced into vessel 145 via line 14-2. Into this vessel is also fed the diluent from 126 passing via line 127, 143 and 144.

The filter cake may then be washed with water at the desired temperature and wash water passed into vessel 147 via line 141 and 146.

If washing is employed, the wash liquor is passed to the wash liquor tank 147 or discharged to any other place via line 149. The wash liquor in 147 may be introduced via line 150 and line 151 into the tank 145 to provide the water, in addition to any introduced via 152 into the contactor to make up for water lost chem ically and otherwise in the process and to control concentrations. The concentrations of alkali in vessel 145 may also be adjusted if found necessary by addition of a suitable alkali solution via 152.

It may also be desirable to adjust the concentration of aluminate in tank 145 in order to establish the proper ratio of aluminate to free alkali. This may be formed by making a solution of some of the recovered hydrated alumina filter cake removed via 160. The solution in 145 after adjustment of concentration of alkali is then passed via line 153 and 154, heat exchanger 122 and heater 155 and line 156 into etch tank 110.

In order to establish continuity of flow, two parallel tanks 145 and two parallel filters 136 may be provided, although one only of each is illustrated for simplicity.

In carrying out the process according to the above procedure, the rate of withdrawal of the liquor via pump 111 is balanced against the rate of introduction of the liquor via line 156 and the rate of feed and concentration of free alkali and dissolved aluminum in the solution in tank 145 is adjusted, so that considering the rate of solution of the metal in tank 110, a substantially constant concentration of aluminate in solution and free caustic is obtained in tank 110 during processing. These concentrations may be within the range of about 20% of free NaOH and from about 1 to 25% of dissolved aluminum calculated as NaAlO depending on the temperature employed, the alloy treated and the etch rate desired. 'Ihese concentrations are proportioned to give the desired etch rate at the temperatures employed which may vary from 150 to 200 F. or more, as described above.

The following example is given merely in illustration of the above process shown in Fig. 1:

Example 4 Assume that the metal etched is 7075 aluminum alloy and that the metal is dissolving at the rate of 1.00 mil per surface per minute per square foot of metal etched, and the concentration is free sodium hydroxide by weight and about 5% by weight of dissolved aluminum calculated as NaAlO Assume that the solution in tank 110 weighs 10,000 pounds, and that we are adding aluminum alloy at the rate of 17 pounds per hour, i.e., etching sq. ft. of

metal in a tank 'having a free surface area of the liquid in the tank of about 32 square feet, i.e., tank of 4 feet wide and 8 feet long. The amount of dissolved aluminum is thus rising at the rate of 15.5 pounds per hour.

Since the 7075 metal contains about 9% of nonaluminum metal as alloying metals, we are adding about 1.5 lbs. per hour of non-aluminum metal which goes to make up sludge including the zinc which is present as zinc oxide.

The solution is withdrawn at a rate such that, considering the percent of the dissolved aluminum precipitated in the regeneration procedure and the increase in free caustic thus resulting, the solution returning to the tank 110 via 156 will be at the required concentration above 10% and the dissolved aluminum sufficiently below 5% (calculated as NaAlO to maintain a constant equilibrium concentration, of aluminate and caustic, in the solution of 10% free NaOH and aluminate at 5% calculated as NaAlO The smut is separated in the centrifuges 114 and 115 and the clarified solution is passed through the heat exchanger 122' and cooler 123, whereby it is reduced to a relatively lower temperature, for example 50 to F. I also add aluminum hydrate via 134 in amount sufficient to establish an excess of solid phase in the contactor 122. The material is held in the contactor 122 under constant agitation and kept at the required temperature, for example 50 to 120 R, if the temperature is below ambient temperature by cooling coils 125 in the contactor, the dwell time being sutficient to obtain the desired percentage of hydrolysis of the dissolved aluminate, for example, 5 to 24 hours.

The efiluent may then be passed into the settler 132. By proper control of the temperature in 122 and the dwell time and the recirculation of the solid hydrate via 134, I may control the amount of hydrolysis of aluminate and the precipitation in 122.

Water loss occurs in the etch tank, both by the chemical reaction and by evaporation and spray. Water is also lost as water of crystallization of Al O .3H O in the regeneration process and is also lost together with some alkali and aluminate in the liquor occluded in the filter cake. This may be supplied in the wash water added to or via 152.

This weight is replenished by addition of water and free NaOH to reconstitute the weight of etchant removed (not including the aluminum introduced into the solution by the etching reaction) and also to provide the concentration of free NaOH and dissolved aluminum and their ratio needed to maintain constant conditions in the etch tank. This water and free NaOH is added in the doctor tank 145. If the precipitation of hydrated alumina in the regeneration process has carried the level of the aluminate in the regenerated solution to such a low value as to cause dilution of the concentration of the aluminate in tank 110 on addition of the material via 156, I add sodium aluminate to the tank 145 as described above.

Instead of operating as stated above, the conditions in tank 110 may be such as to cause sludge precipitation. Such a condition may occur, in a continuous process wherein it is desired for purposes of controlling the etch rate of the metal to maintain a relative high concentration of aluminate in solution such that at the temperatures and alkali concentration the aluminate formed by etching is spontaneously hydrolyzed in solution to precipitate alumina hydrate or the aluminate is present in excess of its solubility. Thus either hydrated alumina and aluminate is precipitated or aluminate alone is precipitated depending on the temperature, and concentrations of free and combined alkali in the aluminate solution maintained in the etch tank.

Another condition which may be encountered, par ticularly in batch operations, occurs where the etch is carried out by employing in tank 110 alkali such as NaOH at a relatively high concentration, for example, 10-20% NaOH and etching aluminum in the bath. Thus, more and more aluminum dissolves in the bath, the alkali is consumed to form aluminate and the concentration of aluminate increases. To compensate for this loss of alkali more alkali is added and the batch is continued in use. A point is reached where due to the accumulation of aluminate in solution, the concentration of alkali required to maintain a desirable etch is excessively high so that the batch is discarded.

The solution in the bath is high in aluminate and may if the aluminate content is sufiiciently high and the use of the bath sufficiently prolonged form sludge which is composed of hydrated alumina and may also contain precipitated aluminate.

In the case where the etch process is discontinued prior to the formation of any aluminatecontaining sludge, and in the event the aluminate content is such that aluminate does not precipitate in the regeneration stage, I may process the withdrawn etch solution as described above and employ the material in tank 145, properly reconstituted to form a new solution in the etch tank 110.

Where in either a continuous process or a batch process, sludge is formed in the etch tank which contains hydrated alumina together with precipitated aluminate, the centrifuge will remove the total sludge. The cake on the centrifuges will be composed largely of hydrated alumina and non-aluminum metal compounds. If the aluminate is present, the cake on the centrifuges may be washed with water to leach out the aluminate. The wash liquor may be added to the clarified material entering 122 via line 119. By controlling the concentration of the aluminate in the wash water, it will in this manner be possible to increase the concentration of aluminate in the solution entering the regeneration stage and thus also the ratio of the aluminate to the free alkali. The nonaluminum metal values in the sludge on the centrifuges may be recovered by treating the cake, also containing hydrated alumina, with a hot caustic soda solution to dissolve the hydrated alumina, leaving the non-aluminum metal sludge or compounds as solid residue recoverable by filtration or centrifuging.

The solution passing via 119 is then cooled and passed into 122 and 132 and processed in the same manner 'as described above.

The centrifuges may, however, be by-passed and the sludge and etch solution passed directly to the regeneration stage for circulation through the contactor 122 and thickener 132. The recirculated hydrated alumina via 134 may not be the thickened slurry withdrawn via 135 as previously described but the reslurried washed filter cake in tank 158 passed through line 159 into line 131' and through the cooler 123 to adjust the temperature.

The settled slurry from 132 containing both hydrated alumina and aluminate precipitate is filtered in 136 as previously and the effluent from 126 passed to tank 145. The filtrate is passed to 145 as previously. The wash liquor from 136 containing dissolved aluminate produced by washing the filter cake is introduced into slurrying tank 158 where it is mixed with a portion of the washed filter cake in tank 158. The residual unused portion of the washed filter cake is conveyed via conveyor 160 to storage. The wash liquor is divided and that part that is suitably high in aluminate is passed into the slurrying tank, and where the concentration falls below the desir- 10 able limit, it may be passed via line 141 to the tank 147 or passed to a suitable disposal via 149.

While I have described a particular embodiment of my invention for the purpose of illustration, it should .be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

I claim:

1. A method of etching aluminum comprising, contacting aluminum with a solution containing free alkali metal hydroxide at a relatively high temperature to react the aluminum with said alkali metal hydroxide and form alkali metal aluminate in a first zone without producing any substantial amount of solid hydrated alumina therein, removing the resultant solution to a second zone, cooling said removed solution in said second zone in the presence of solid hydrated alumina to precipitate an additional quantity of solid hydrated alumina therefrom and increase the quantity of free alkali metal hydroxide therein, separating the precipitated hydrated alumina from the solution, and returning the solution to the first zone.

2. A method of etching aluminum as set forth in claim 1, wherein alkali metal aluminate precipitated with said hydrated alumina is dissolved out of the separated precipitate and returned to the solution.

3. A method of etching aluminum as set forth in claim 1, wherein in the solution in the first zone the ratio of total alkali, expressed as mols of alkali metal oxide, to aluminum in solution, expressed as mols of A1 0 is maintained in excess of about 1.1.

4. A method of etching aluminum as set forth in claim 3, wherein said ratio is maintained in excess of about 1.5.

5. A method of etching aluminum as set forth in claim 4, wherein the temperature of the solution in the first zone is maintained in the range of about -190 F.

6. A method of etching aluminum as set forth in claim '3, wherein the temperature of the solution in the first zone is maintained in the range of about 170-190 F.

References Cited in the file of this patent UNITED STATES PATENTS 1,131,067 Landreth Mar. 9, 1915 1,716,270 Holmstrom June 4, 1929 2,235,658 Waterman Mar. 18, 1941 2,522,605 Cundifi Sept. 19,1950 2,650,875 Dvorkovitz Sept. 1,1953. 2,775,508 Thomsen Dec. 25, 1956 f 

1. A METHOD OF ETCHING ALUMINUM COMPRISING, CONTACTING ALUMINUM WITH A SOLUTION CONTAINING FREE ALKALI METAL HYDROXIDE AT A RELATIVELY HIGH TEMPERATURE TO REACT THE ALUMINUM WITH SAID ALKALI METAL HYDROXIDE AND FORM ALKALI METAL ALUMINATE IN A FIRST ZONE WITHOUT PRODUCING ANY SUBSTANTIAL AMOUNT OF SOLID HYDRATED ALUMINA THEREIN, REMOVING THE RESULTANT SOLUTION TO A SECOND ZONE, COOLING SAID REMOVED SOLUTION IN SAID SECOND ZONE IN THE PRESENCE OF SOLID HYDRATED ALUMINA TO PRECIPITATE AN ADDITIONAL QUANTITY OF SOLID HYDRATED ALUMINA THEREFROM AND INCREASE THE QUANTITY OF FREE ALKALI METAL HYDROXIDE THEREIN, SEPARATING THE PRECIPITATED HYDRATED ALUMINA FROM THE SOLUTION, AND RETURNING THE SOLUTION TO THE FIRST ZONE. 